The driving power for a hydroelectric generation installation typically comprises a reservoir of water created by the construction of a dam across a river or other waterway system, at least one electric generator driven by a turbine receiving a channeled flow of headwater from the reservoir, and a downstream discharge system for egress of spent water from the turbine back into the river or waterway system. The amount of electricity generated in such installations is directly affected by the height of the water in the dam at the water intake stand pipe that feeds the penstock delivering the water to the turbine. In general terms, increasing the height of a dam during construction enables the installation of a taller water intake stand pipe thereby enabling the delivery of a taller column of water to the turbine. The bottom of the water column is typically defined by the bottom surface of the reservoir. It is the pressure caused by weight of the volume of water during its vertical drop from top to the bottom of the column that determines the force with which the turbine is driven. Turbines are commonly situated at or near the bottom of the water column. The vertical drop of water provided to the turbine is commonly referred to as the “head” of the dam. The water pressure delivered to the turbine can be manipulated by the diameters selected for the intake stand pipe and the penstock, and by the height at which the water inlets into the intake stand pipe are positioned relative to the turbine to which the water is delivered. Maintenance of adequate volumes of water in dam reservoirs is dependent on the rate of water flow in the upstream waterway feeding into the dam, and on the annual precipitation in the forms of rain, snow and snowmelt that supply water into the upstream waterway. Extended periods of peak power production are accompanied by high discharge volumes of spent water from the dam tailraces into the tailrace pools adjacent the dam often result in the downstream water levels in the tailrace pools rising above the levels of the turbine installation thereby directly causing a decrease in power production. Those skilled in these arts understand that the distance of the water column from the uppermost intake of the stand pipe to the downstream water level in the tailrace pool adjacent the dam, where backpressure becomes effective is known as the “net head” of the dam and that the “net head” is the primary determinant of the driving force delivered to the turbine and therefore, the height of the “net head” directly affects hydroelectric power generation. Multiple vertically positioned water inlets are provided on individual stand pipes to enable delivery of water to the turbine when the water level in reservoir drops during periods of extended dry and/or drought conditions. However, it is known by those skilled in these arts that the levels of impounded water maintained in a hydroelectric dam reservoir are directly affected by power production by the dam, i.e., by the rates of water removal from the reservoir and delivery to the turbines.
Another problem associated with hydroelectric power generation during periods of low power generation when the turbines are idling, or during extended dry or drought periods, is that reduction of water pressure into the turbine results in a lower volume of water egress from the tailrace into the downstream waterway. A common consequence is that the water levels in the downstream water decline to the point where a portion or all of the tailrace is exposed to the atmosphere thereby allowing air to ingress into the turbine via the tailrace infrastructure, predisposing the turbine to cavitation within the water delivery-egress infrastructure. The high-speed rotation of the turbine blades intermixes the air from the tailrace with the ingressing headwater from the reservoir causing a plurality of localized intense low-pressure regions (i.e., vacuum pressure) comprising air bubbles swirling about the turbine blades and shaft. The bubbles tend to collapse violently sending out shock waves that physically impact surround solid surfaces, initially causing minor damage in the form of pits and abrasions in the blade surfaces that over extended periods or episodes of cavitation, may increase in size to form voids within the blades and to cause fatigue in the materials used to configure the blades and the turbine shaft. Such damaged turbines must be replaced to prevent serious equipment malfunctions and breakdowns which may incapacitate the hydroelectric generating plant until repairs are made.
The prior art discloses several strategies for increasing the effective head of a dam without having to increase the height of the dam and for controlling the level of water in a tailrace in response to seasonal water flow fluctuations upstream of the dam. U.S. Pat. No. 4,014,173 discloses installing a generator-driving turbine in a water-tight pit that is substantially below the bed level of the tail water and continuously removing water discharged from the turbine by a self-energizing impulse pump known to those skilled in these arts as a hydraulic ram. The hydraulic ram is preferably installed in a second water-tight pit located downstream from the turbine pit and must be interconnected to the turbine pit by an underground piping infrastructure. The complex construction required by the '173 system not suitable for many landscapes which are suitable for installation of hydroelectric power generation stations; furthermore, this system is difficult to retrofit to existing hydroelectric generating installations. Furthermore, the design and the configuration of the turbine and hydraulic ram pits impose restrictions on ease-of-access for maintenance, repair and replacement work. GB 700,320 discloses an adjustable weir installed in the tailrace of a hydraulic turbine during construction of the hydroelectric generating dam for the purpose of maintaining a level of water in tailrace sufficiently high so as to prevent cavitation at the turbine. The adjustable weir is automatically controlled by a device responsive to the water level in the tailrace in such a manner that the weir is kept below and out of the flow of water egressing from the turbine as long as the water level does not drop below a pre-determined minimum level. If the water flow does drop below the minimum level, then the adjustable weir is raised to dam up the water in the tailrace thereby raising the water level above the minimum required to prevent cavitation. The '320 adjustable weir is designed to be continually submerged in the tailrace water flow and consequently is subject to numerous operational problems including: (1) propensity for failure of individual components of the adjustable weir or of the unit itself as a consequence of wear and corrosion from being continually submerged, (2) difficulty of access for under-water maintenance and repair, and (3) the '320 weir is not debris-tolerant, i.e., any back-washed bottom scour such as rocks, tree limbs and other water-logged debris will jam against the weir and interfere with its operation.